1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine.
2. Description of Related Art
A spark plug, which is used as an ignition means for an internal combustion engine, includes an insulator holding a center electrode inserted therein, a housing holding the a distal end portion of the insulator inserted therein, and a ground electrode joined to the housing so as to forma spark discharge gap with the center electrode. Insulation of the spark discharge gap is broken to generate a discharge spark between the center electrode and the ground electrode when applied with a high voltage from an ignition coil.
Japanese Examined Patent Application Publication No. H6-80313 describes such a spark plug. The spark plug described in this patent document includes a constant-voltage element which is incorporated in the insulator. When a voltage applied to the constant-voltage element is lower than a set voltage, a current is prohibited from flowing through the constant-voltage element. On the other hand, when the voltage applied to the constant-voltage element is higher than or equal to the set voltage, the resistance of the constant-voltage element is sufficiently reduced to allow a current to pass therethrough. The provision of the constant-voltage element parallel-connected to the spark discharge gap makes it possible to prevent the spark discharge gap from being applied with a voltage higher than the set voltage to thereby reduce variation of the discharge voltage.
However, the spark plug described in the above patent document has a problem as described below. Internal combustion engines with a small-sized supercharger are increasing in number to address the demand of improvement of fuel economy and reduction of the manufacturing cost. In these internal combustion engines, the compression ratio is set higher causing the cylinder pressure to be higher and causing the discharge voltage of the spark plug to be higher.
Further, the center electrode and ground electrode wear more because of increase of the temperature of the combustion chamber, increase of material oxidization in oxidization atmosphere, volume reduction due to spark sputtering and the like. As a result, since the spark discharge gap between the center electrode and the ground electrode increases, the discharge voltage of the spark plug further increases.
Hence, for the spark plug provided with the constant-voltage element to prevent the spark discharge gap from being applied with a voltage higher than the set voltage, there is a concern that it may take a long time before a discharge spark is generated or a discharge spark may not be generated causing a misfire of the internal combustion engine.